Antenna system comprising driver circuits for transponder

ABSTRACT

A transponder for, for example, domestic animals can receive high frequency energy and transmit information of an antenna (L x , L y , L z ) and has a storage capacitor (C 5 ) for storing received energy, which is used for transmitting on the antenna. In order to make the antenna less sensitive to the geometric direction of the received high frequency energy, it has a plurality of antenna coils (L x , L y , L z ), that are arranged in an angle to each other, e.g. perpendicularly to each other. Monitoring circuits (LD x , LD y , LD z , 1) can detect the energy received on each coil and compare the detected values to each other and can, for issuing information, use that coil on which the largest energy has been received. Thereby, the transmission can be made in a direction which is considerably more optimal than a direction which otherwise would be obtained in a more random fashion. Each antenna portion is further included in an oscillatory circuit having two different natural frequencies. Thereby information of the transponder can be issued by modulating between these frequencies, what produces a transmission that is more free from interference compared to the case where only one frequency is used with on/off-modulation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a transponder, in particular an antennasystem for a transponder and activation or driver circuits incorporatedin the transponder for issuing information carrying electromagneticwaves on the antenna system.

2. Description of the Prior Art

Electronic systems for identification of domestic animals utilizeelectronic units attached to the animals, usually in the shape oftransponders that lack an energy source of their own. Such transpondersobtain induced electric energy from receiving suitable electromagneticenergy that is emitted by antennas or coils located at those positionswhere an identification is desired, such as at the entrance of a feedingplace or a milking station. After receiving induced energy thetransponder issues a radio frequency signal that is captured by anantenna which can be the same antenna by means of which the energy wasinduced or at least is arranged in parallel therewith. Theidentification signal can then be used for e.g. only counting the numberof animals, controlling amounts of delivered food, guiding a milkinganimal to the correct stall, etc.

Transponders are generally used for identification of movable objects,for example and primarily of bovine animals. A transponder comprisesconventionally a receiving and transmitting antenna, a storage means forstoring energy transferred wirelessly to the transponder and controlcircuits. A transponder is associated with at least one usually fixed orstationary reading station that comprises an antenna loop having one ormore turns located at some position, in the direct neighbourhood ofwhich the movable object is sometimes located or passes, such as forexample directly through the antenna loop of the reading station. Theantenna of the transponder is in a similar way made as a coil having amultitude of turns, the coil being substantially flat. The antenna ofthe transponder and the antenna in the reading station cooperate inprincipally the same way as the two windings of a transformer having anair core. By suitable arrangements it can often be achieved that thesetwo coils at least sometimes will be located approximately in parallelwith each other, when the movable object is staying close to or passesthe stationary reading station, but this can be difficult to achieve ina secure manner, e.g. for the case where the reading station comprisesloops located at each side of a passage through which the movableobjects pass. Thus, it would be advantageous that the antenna systemcomprising the two antennas could be arranged for arbitrary mutualdirections of the magnetic field of the stationary antenna in thereading station and the antenna of the transponder.

U.S. Pat. Nos. 4,247,758 and 4,798,175 and the published European patentapplications EP-A1 0 494 764 and EP-A2 0 496 610 disclose typicalexamples of prior transponder systems and antennas designed therefor.

In U.S. Pat. No. 4,274,089 it is disclosed how an interrogation systemfor theft protection responders can be designed to have two transmissionantennas, each one of which being a flat coil and which are locatedperpendicularly to each other. These antenna coils are made having onlyone terminal and thus they have one end open. They are supplied withelectrical energy alternatingly so that always only one antenna isactive. A separate receiving antenna is used. By the arrangementcomprising antenna coils arranged perpendicularly to each other thestation is less sensitive to the orientation of the antenna of aresponder attached to some object that passes the interrogation station.

This previously known design having two antenna elements arrangedperpendicularly to each other can of course also be modified for use inreading stations for transponders, as appears from the publishedEuropean patent application EP-A1 0 496 609. Here two vertical antennacoils that are perpendicular to each other are provided surrounding thepath along which a transponder passes. U.S. Pat. No. 5,258,766 disclosesa similar way of accomplishing that the antenna system in atransponder/reading station installation will have the quality mentionedabove, i.e. that a good transfer of energy and of information isobtained for arbitrary angles of the two antennas. The reading stationis here provided with three antenna loops located perpendicularly toeach other. In addition, each antenna loop consists of two parallelportions located at a distance from each other so that a cubical orcomplex, ball shaped arrangement is obtained, where in the cubicalarrangement antenna conductors are located at all edge lines of thecube.

Antennas of reading stations are usually made in the shape of a portalstructure, see for example the patent U.S. Pat. No. 4,798,175 citedabove. The electrical conductor in the antenna here forms loops locatedat each side of a passage through which the animal are intended to pass.Such a portal structure could be completed to comprise an antennaarrangement that is previously known according to the discussion abovehaving antenna loops located above and underneath the passage andfurther having loop elements through which the animal must pass, inorder to produce a more secure transfer of energy and more securereceiving of information. However, the antenna arrangement must be madevery sturdy or robust having the electrical conductors of the antennaattached to some electrically isolating material, that in the priordesign comprises curtains suspended at the sides of the passage andconsisting of thick plastic plates. The plastics material must here beselected to have such a thickness and such a quality that it is opaque,and thus the portal will have the appearance of a dark passage or darkopening. Bovine animals can be afraid of such a dark passage and specialarrangements must be made for making them pass through the portal. Ifthe prior portal is completed to comprise further parts it would be evenclumsier and heavier and it must be designed to include rigid steelparts provided with a suitable electrical isolation. For a readingstation for bovine animals where it is desired that the animals to whichthe transponders are attached at their passage preferably are to passthrough or close to an antenna loop of the reading station, the antennaloop should thus be designed in the simplest possible way, having themost possible open configuration. Apparently, such an open configurationcannot be obtained using the prior antenna systems.

A transponder comprises conventionally an oscillatory circuit, in whichthe antenna coil and a capacitor are included and which is coupled to acapacitor for storing energy, see the published European patentapplication EP-A1 0 301 127 and the German patent documents DE-C1 40 04801 and DE-C1 40 04 196. The oscillatory circuit is tuned to thefrequency of the electromagnetic energy that is issued from a readingstation together with which the transponder is intended to be used, andthe oscillatory circuit is through some suitable rectifying devicefurther coupled to a capacitor for storing energy. When sufficientenergy has been received and stored in the storage capacitor, itprovides supply voltage to control circuits so that information can beissued from the transponder antenna, generally having the same frequencyas the received electromagnetic energy. Then the oscillatory circuit canbe made to oscillate at its natural or resonant frequency by connectingit to the electrodes of the storage capacitor during short periods atsuitable times, see the European patent application EP-A1 0 301 127cited above. The times can be selected so that they occur at everysecond cycle of the natural oscillation of the oscillatory circuit, seethe divider circuit 158 and the monostable flipflop 192 in FIG. 2.Information from the transponder is then emitted by modulating thetransmitted wave in an off-and-on fashion. In order to obtain a safereception it is required that the reading station issues the highfrequency energy in intervals or periodically, so that after atransmission interval a transponder that is located within a certaindistance from the antenna reading distance normally has had its storagecapacitor charged to a sufficient voltage in order to drive its controlcircuits and in order to be able to transmit information.

In another design the information issued from the transponder can becarried by an electromagnetic oscillating wave having a frequencydifferent from that of the received high frequency energy, see theGerman "Offenlegungsschrift" DE-A1 42 13 065. Thereby possibly highfrequency can be transmitted more continuously and perhaps longertransmission periods of the transponder can be obtained.

However, a disadvantage in these known designs is that in emittinginformation from the transponder the modulation is made by means of an"on/off" process which is known to be sensitive to interference and canrequire several retransmissions of information in order that it will besafely received.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a transponder and an antennasystem for a transponder achieving that the reading of the transponderis secure when it passes a reading station.

It is another object to provide a transponder and an antenna for atransponder that can be used for arbitrary locations and directions ofthe antenna in a reading station.

A problem that the invention intends to solve is according to thediscussion above to provide a transponder and an antenna system for atransponder which can be used together with existing antennas in readingstations without having to change these antennas and which achieve agood transfer of electromagnetic energy to and of information carried byelectromagnetic waves from the antenna of the transponder.

It is another object of the invention to provide a transponder that canperform transmission of information in a way that can be securelydetected by a reading station.

Another problem that the invention intends to solve is thus to provide atransponder which issues electromagnetic radiation in such a way thatthe information content of the radiation can be securely detected by areceiving station.

A transponder for e.g. domestic animals can in the usual way receivehigh frequency energy and transmit information on an antenna and has astorage capacitor for storing received energy that is used whentransmitting on the antenna. In order to make the antenna less sensitiveto the geometric direction of the received high frequency it has aplurality of antenna coils located in an angle to each other, forexample perpendicularly to each other. Each such antenna coil operatesindependently of the other antenna coils and is thus tuned to thefrequency of the wirelessly received high frequency energy, or worded ina more accurate way, is included in an oscillating circuit tuned to thisfrequency. This means that the natural oscillation frequency or theresonance frequency of the oscillatory circuit approximately agrees withthe frequency of the high frequency energy. Further, each antennaportion is connected to a storage capacitor for storing high frequencyenergy, the storage capacitor advantageously being common to all antennaportions.

Monitoring circuits can detect the energy received on each coil andcompare the detective values to each other, and can, for transmittinginformation, use that coil on which the largest energy has beenreceived. Thereby the transmission can be made in a direction that issignificantly more optimal than a more random direction otherwiseobtained.

The antenna arrangement of the transponder is geometrically similar tothe antenna system of the reading station according to the Europeanpatent application 0 496 609 cited above. However, it is significantlysimpler than the more complicated ball configuration that also accordingto the discussion above is known for the stationary reading station. Thelatter antenna configuration thus comprises two loop portions parallelto each other for each perpendicular direction, whereas the antenna ofthe transponder only has to comprise simple, substantially flat loops,which are located perpendicularly and symmetrically in relation to eachother, so that a simpler or more genuine ball configuration is obtained.

In the German patent application known through "Offenlegungsschrift"DE-A1 42 13 065 a self identifying telemetric device is disclosed, thatin the discussion of the problem presented in that application isproposed to comprise two antenna portions L₀₁ and L₀₂ which are arrangedperpendicularly to each other. However, these two antenna portions havedifferent resonant frequencies and are thus intended for receivingelectromagnetic radiation of different wavelengths. Apparently such aconstruction does not improve the reception for different angularpositions of the transponder antenna for a considered wavelength. Inthis prior design further the antenna portions are joined by means of aferrite core, whereas in the antenna arrangement for a transponder,which has been described above, no core elements are required.

A transponder for receiving wirelessly transferred high frequency energyand for transmitting wirelessly information thus comprises an antennafor receiving and transmitting, an energy storage unit connected to theantenna for storing energy and a control and transmission unit which isarranged to transfer an information signal to the antenna, so that thesignal is emitted from or on the antenna by means of energy stored inthe energy storage unit. In order to achieve better receiving and inparticular better transmitting the antenna comprises at least twoseparate antenna portions that each one is arranged for wirelessreceiving and transmitting and each one in the conventional way has adirection in which the efficiency in receiving is at a maximum. Thesedirections are arranged in considerable angles in relation to eachother, in particular substantially 90°, and should always be comprisedwithin the interval between 60° and 120°. In order to achieve a suitablesymmetry preferably three such separate antenna portions are providedhaving directions for maximum receiving and these directions are alloriented in substantially the angle of 90° in relation to each other.

Each antenna portion can in a simple embodiment comprise a winded coil,the winding turns of which all are located in planes that areessentially parallel to each other. Then the central plane of eachwinded coil exists, so that a winding turn is always located at eachside of or in the central plane. Further, an equal number of windingturns can be arranged at each side of the center plane and the coil canbe substantially flat, i.e. the depth of each coil perpendicular to thecenter plane can be small compared to the length and width of the coil,in particular be less than 1/5 of the length and width of the coil.

Each antenna portion can comprise a winded coil, the winding turns ofwhich have substantially a circular or rectangular, in particular asquare, shape.

In order to be able to control receiving and transmitting to a certainantenna portion or coil which is located in the best way in relation toan exterior, stationarily arranged antenna with which the transpondercooperates, a level detector can be provided for each separate antennaportion and it then provides suitably a signal to the control andtransmission unit which represents some suitable intensity measure, forexample the amplitude, of the oscillation received in the antennaportion. The control and transmission unit then comprises advantageouslycomparing means connected to the level detectors for comparing thesignals therefrom and control means that ensure that in transmittingfrom the transponder that antenna portion is used for which thecomparing means indicate or have indicated that the largest amplitudeexisted in receiving that electromagnetic wave on the antenna whichcontains the energy which is to be used for the transmission. Thecontrol means can further ensure that in transmitting from thetransponder also that antenna portion is used, for which the comparingmeans indicate that next to the largest amplitude existed in receivingthe energy used for the transmission.

Oscillatory circuit capacitors are suitably connected in parallel witheach antenna portion for forming an oscillator and filter circuittogether with the antenna portion and the inductance thereof. Eachantenna portion is in that way included in a filter and oscillatorycircuit of its own. Therein a self generated oscillation can be startedhaving an own natural or resonant frequency by applying a voltage pulseover the circuit, where the energy of the voltage pulse is taken fromthe energy storage unit. The control and transmission unit can thenperiodically apply voltage pulses over the filter and oscillatorycircuit having a frequency that is substantially equal to an evenfraction of the natural frequency, whereby an oscillation having afrequency substantially equal to the natural frequency can be maintainedin the filter and oscillatory circuit.

By arranging components having variable characteristics such asoscillatory circuit capacitors having variable capacitances or by thefact that the capacitors each one comprises a plurality of capacitorsthat can be connected in a controlled way, the filter and oscillatorycircuits can be made controllable to a plurality of different naturalfrequencies, in the preferred case two different natural frequencies.The control and transmission unit can then as above control theapplication of voltage pulses to the oscillator and filter circuits withfrequencies which are substantially an even fraction of each one of thedifferent natural frequencies, dependant on the frequency that isinstantaneously valid. An even fraction is here taken to mean that thehigher frequency corresponds essentially to an integer multiple of thelower frequency.

The filter and oscillatory circuit can for arranging two differentnatural frequencies comprise a first capacitor that is directly andpermanently connected in parallel with the antenna portion or the coil,and a second capacitor, which is arranged to be connected, in acontrolled way by applying a control signal from the control andtransmission unit, in parallel with the first capacitor.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail with reference tonon limiting, particular embodiments and with reference to theaccompanying drawings, in which

FIGS. 1a and 1b are schematic perspective views of two differentembodiments of an antenna system for a transponder,

FIG. 2 is a circuit diagram of electronic circuits to be used togetherwith the antenna system of FIGS. 1a or 1b.

DESCRIPTION OF PREFERRED EMBODIMENTS

In FIG. 1a an antenna system for a transponder is illustrated thatcomprises three separate antenna windings, an antenna winding L_(x), thewinding turns of which all are located in parallel with an x-coordinateplane, an antenna winding L_(y), the winding turns of which all arelocated in parallel with a y coordinate plane, and an antenna windingL_(z), the winding turns of which are located in parallel with az-coordinate plane. Each antenna winding L_(x), L_(y), L_(z) comprises anumber of turns of an isolated electric conductor, which are locatedclose to each other, so that a coil is formed having a rather smallheight or width. Each coil has a symmetry plane that passes through thecentral winding turns in the coil. Further, from the antenna windingsthe ends of the electric conductors continue in the shape of connectorconductors t_(x), t_(y), t_(z), to be connected to electronic circuitswhich will be described hereinafter. Each winding L_(x), L_(y), L_(z)has in the embodiment illustrated in FIG. 1a a rectangular shape, forexample a square shape, whereas according to the embodiment of FIG. 1beach winding is substantially circular. In the latter figure theconnector wires are not shown. What is essential for the function isthat each winding is substantially identical and that the three coilsare located substantially perpendicularly to each other, though rathergreat deviations may be allowed from this configuration. In theembodiments shown in FIGS. 1a and 1b also said symmetry plane extendingthrough the central winding turns in an antenna winding is anothersymmetry plane of each one of the other antenna windings, this symmetryplane being perpendicular to the corresponding antenna coil. In the caseof square antenna coils these other symmetry planes extend through thecenter of opposite sides of the square configuration.

When an antenna of the kind illustrated in FIGS. 1a and lb passesthrough or at an antenna of a transponder reading station, not shown,which normally issues a wave of a suitably high frequency, in each oneof the windings L_(x), L_(y), L_(z) voltages having different magnitudesare induced. In order to use these different voltages the connectorwires t_(x), t_(y), t_(z) are coupled to suitable circuits shown in thecircuit diagram of FIG. 2.

Thus, in parallel with each antenna coil or winding L_(x), L_(y), L_(z)capacitors C_(x), C_(y), C_(z) are connected, so that the antennawinding together with the capacitor forms a suitable oscillatory circuitor a corresponding band pass filter tuned to the frequency of theelectromagnetic wave emitted by the interrogation station. One side orone terminal of the filters or the oscillatory circuits formed by theantenna loops together with corresponding capacitors are connected tothe first side or the first electrode of a storage capacitor C_(s) andthe other side of the oscillatory circuits are through diodes D_(x),D_(y), D_(z) connected to the second plate or electrode of the storagecapacitor C_(s). Thereby a storage capacitor C_(s) will be charged ateach half period of an induced voltage which has one of the possiblepolarities, up to a peak voltage which is equal to the amplitude of thelargest induced AC voltage.

Further, the two electrodes of the storage capacitors C_(s) areconnected to a central control and transmission unit or control circuit1, which comprises suitable logical circuits and which obtains itssupply voltage exactly from these terminals of the storage capacitorC_(s). The connection points of one terminal of the antenna windingsL_(x), L_(y), L_(z) and the diodes D_(x), D_(y), D_(z) are throughdecoupling capacitors C_(Kx), C_(Ky), C_(Kz) connected to leveldetectors LD_(x), LD_(y), LD_(z). The decoupling capacitors have asuitably adapted capacitance, so that they work substantially as shortcircuits for the AC voltage induced in the oscillatory circuit, which isthus transferred to the inputs of the level detectors. The leveldetectors LD_(x), LD_(y), LD_(z) sense in some known manner theamplitude of the electric voltages received in the antenna coils L_(x),L_(y), L_(z), for example their rectified mean or peak value, and thusdelivers DC voltage signals on their outputs through connection lines tothe control circuits 1. The control circuits evaluate these signals anddetermine the antenna coil L_(x), L_(y), L_(z), which provides thelargest input signal.

For issuing information from the transponder, according to a preferredembodiment, only that one of the antenna coils L_(x), L_(y), L_(z) whichhas been determined to give the largest induced voltage from the readingstation is used. Therefor first semiconductor switches S_(1x), S_(1y),S_(1z) are connected in parallel with the diodes D_(x), D_(y) and D_(z)respectively, for example suitable field effect transistors. These firstswitches thus have a first interruption terminal coupled to one side ofthe oscillatory circuits and a second interruption terminal coupled tothe second electrode of the storage capacitor C_(s). Further, secondconductor switches or transistor switches S_(2x), S_(2y), S_(2z) areconnected in the connection line between the decoupling capacitors andthe second side of the storage capacitors C_(s), i.e. in parallel withthe first semiconductor switches. The gates of the switches are allconnected to the control circuits 1.

When transmitting from the transponder that one of the first switchesS_(1x), S_(1y), S_(1z) is used, which is connected to the antennawinding L_(u), u=x, y or z, that has given the largest input signal, byapplying a suitable voltage to the gate of this switch. The chargecapacitor C_(s) is then discharged through the antenna coil L_(u), i.e.a voltage pulse is obtained over the oscillatory circuit, the energybeing drawn from the storage capacitor C_(s). Thereby a naturaloscillation is started in the electric oscillatory circuit formed by theantenna winding L_(u) and its associated capacitor C_(u), the frequencyof the resonant oscillation being equal to the resonant frequency of thecircuit. Such activation pulses, produced by closing the switch S_(1u)for short periods, can be applied approximately in the same rhythm asthat of the oscillation rhythm but these pulses need only to occur in alower frequency, so that the activation pulses occur only approximatelyat each m:th pulse of the resonant oscillation, where m is an integerlarger than 1, for example equal to four. The integer m can be a not toolarge integer such as some integer in the interval of [3, 16] having amost preferable value in the interval [3, 8].

Further, that switch S_(2u) among the second switches S_(2x), S_(2y),S_(2z), which is associated with the activated antenna winding L_(u),can also be closed during the forced oscillation, by applying a suitablesignal to its gate terminal from the control circuit 1. The decouplingcapacitor C_(Ku) will then be connected or seem to operate in parallelwith the capacitor C_(u) in the oscillatory circuit due to the fact thatthe storage capacitor C_(s) has a large capacitance and works as a shortcircuit for the actual oscillator frequencies. When the decouplingcapacitor is connected, the capacitance in the oscillatory circuit willbe larger and a lower natural frequency is obtained compared to the casewhere it is not connected. However, it can be selected, so that it stillis approximately an even multiple of the frequency with which theactivation pulses are applied, e.g. equal to three times that frequency.In that way the oscillatory circuit can in a simple way be arranged tohave two different resonant frequencies. For the examples given abovecomprising natural frequencies corresponding to four and three times thefrequency of the activation pulses, it is obtained that the capacitanceof the decoupling capacitors should be one third of the capacitance ofthe capacitors connected in parallel with the antenna coils.

From the antenna coil L_(u) in the resonant oscillation of the selectedoscillatory circuit electromagnetic high frequency energy is issued,which can be detected by the stationary antenna installation with a goodefficiency since it is issued by the antenna coil that with the largestintensity has received energy from the antenna of the stationaryinstallation and therefore with the largest possibility also willprovide the emitted energy which is most easily detected. The emittedenergy can further, according to the discussion above, be given todifferent frequencies by a periodic activation of the first switch withor without the second switch in a closed position. It will in additionprovide a detection of logical transitions between two levels, i.e.between two frequencies, what is significantly more secure than the casewhere only one frequency is used.

The control logical circuits 1 can also determine a priority order ofthe coils with the corresponding intensity values of the energy receivedthereon. Then, also emission from all coils can be made and the emittedamounts of energy can be controlled corresponding to the determinedintensity values. However, it can require a considerably more complexcontrol scheme. A simpler alternative can be to activate only those twooscillatory circuits, for the antenna coils of which the largestreceived intensity values have been determined, and that this activationis made with the same intensity for the two selected oscillatorycircuits.

The control circuits 1 comprise as their main portion a logical controlmeans 3 which contains a suitable control program and further pulsegenerating circuits 5 connected to the switches S_(1x) -S_(2z) fordelivering control signals thereto. The pulse generating circuits 5 cancontain an own oscillator, not shown, and suitable dividing circuits inorder to obtain different suitable pulse frequencies for control of theoscillation in the oscillatory circuits of the antenna portions. Thecontrol means also receives a control signal from a comparator 7, towhich the signals from the level detectors LD_(u) are provided. Thecontrol signal indicates the order of magnitude of the energy receivedin the different oscillatory circuits. The control means 3 is alsoconnected to a data unit 9 that can contain suitable memories andpossibly a measuring unit for generating information of interest that isto be transferred to a reading station.

What is claimed is:
 1. A transponder for receiving high frequency energywhich is wirelessly transferred and for wirelessly transmittinginformation, comprising:an antenna for receiving and transmitting; anenergy storage unit connected to the antenna for storing high frequencyenergy which is transferred wirelessly to the transponder and isreceived on the antenna; and a control and transmission unit fortransferring an information signal to the antenna to be transmitted fromthe antenna using energy stored in the energy storage unit, wherein theantenna comprises at least two separate antenna portions, each one ofwhich is arranged for wireless receiving and wireless transmitting, hasa direction in which the efficiency for receiving is at a maximum andworks independently of the other antenna portions, the directions beingarranged in angular relation to each other.
 2. A transponder accordingto claim 1, wherein said antenna includes three separate antennaportions, each having a direction for maximum receiving, which are alloriented at substantially 90° angles in relation to each other.
 3. Atransponder according to claim 1, wherein each antenna portion comprisesa winded coil having winding turns, the winding turns of each windedcoil being located in planes which are substantially parallel to eachother.
 4. A transponder according to claim 3, wherein a center plane isarranged for each winded coil, so that the winding turns of each windedcoil are located at one of the sides of or in the center plane of thewinded coil and so that an equal number of winding turns are located oneach side of the center plane, and the depth of each winded coil takenperpendicularly to the center plane of the winded coil is small comparedto the length and width of the winded coil.
 5. A transponder accordingto claim 4, wherein the depth of each winded coil is less than 1/5 ofthe length and width of the winded coil.
 6. A transponder according toclaim 1, wherein each antenna portion comprises a winded coil havingwinding turns, the winding turns of each winded coil havingsubstantially circular shapes.
 7. A transponder according to claim 1,wherein each antenna portion comprises a winded coil having windingturns, the winding turns of each winded coil having substantiallyrectangular shapes.
 8. A transponder according to claim 1, furtherincluding a level detector for each separate antenna portion, whichlevel detector is arranged to provide a signal to the control andtransmission unit which signal represents the amplitude of anoscillation obtained when receiving in the antenna portion highfrequency energy which is wirelessly transferred to the transponder. 9.A transponder according to claim 8, wherein the control and transmissionunit comprises comparing means connected to the level detectors forcomparing the signals provided by the level detectors with each otherand control means for using, when wirelessly transmitting informationfrom the transponder, that one of the antenna portions for which aresult of the comparing in the comparing means indicates that thelargest amplitude existed when receiving the high frequency energy. 10.A transponder according to claim 8, wherein the control means arearranged to also use, when the transponder wirelessly transmitsinformation, that antenna portion, for which result of the comparing inthe comparing means indicates that the next to largest amplitude existedwhen receiving the high frequency energy.
 11. A transponder according toclaim 1, further including a plurality of oscillatory circuitcapacitors, each one of which is connected in parallel with only one ofthe antenna portions.
 12. A transponder according to claim 1, whereineach antenna portion is electrically connected in a filter andoscillatory circuit, each filter and oscillatory circuit being arrangedto have a natural oscillation which has a natural frequency and whichcan be started by applying a voltage pulse over the circuit.
 13. Atransponder according to claim 12, wherein the control and transmissionunit, when wirelessly transmitting information from the transponder, isarranged to periodically apply voltage pulses over at least one of thefilter and oscillatory circuits with a frequency substantially equal toan even fraction of the natural frequency.
 14. A transponder accordingto claim 12, wherein the filter and oscillatory circuits arecontrollable to oscillate at two different natural frequencies.
 15. Atransponder according to claim 1, wherein said angular relation betweensaid antenna portions is substantially 90 degrees.
 16. A transponderaccording to claim 1, wherein each antenna portion is connected to astorage capacitor for storing received high frequency energy.
 17. Atransponder according to claim 1, wherein each antenna portion isconnected in an oscillatory circuit which is tuned to the frequency ofhigh frequency energy which is transferred wirelessly to thetransponder.
 18. A transponder for receiving high frequency energy whichis wirelessly transferred to the transponder for wirelessly transmittinginformation, the transponder comprising:an antenna for receiving andtransmitting; an energy storage unit connected to the antenna forstoring high frequency energy which is transferred wirelessly to thetransponder and is received on the antenna; and a control andtransmission unit for transferring an information signal to the antennato be transmitted from the antenna using energy stored in the energystorage unit; wherein the antenna comprises at least two separateantenna portions, each one of which is arranged for wireless receivingand transmitting, has a direction in which the efficiency for receivingis at a maximum, works independently of the other antenna portions andis connected to a storage capacitor for storing received high frequencyenergy, the directions being arranged in angular relation to each other.19. A transponder for receiving high frequency energy which istransferred wirelessly to the transponder and for wirelesslytransmitting information, the transponder comprising:an antenna forreceiving and transmitting; an energy storage unit connected to theantenna for storing high frequency energy which is transferredwirelessly to the transponder and is received on the antenna; and acontrol and transmission unit for transferring an information signal tothe antenna to be transmitted from the antenna using energy stored inthe energy storage unit; wherein the transponder is arranged towirelessly transmit information by modulating between two differentfrequencies.
 20. A transponder according to claim 19, further includinga filter and oscillatory circuit in which the antenna is electricallyconnected and which is arranged to have natural oscillations which havenatural frequencies and can be started by applying a voltage pulse overthe filter and oscillatory circuit with energy drawn from the energystorage unit, the filter and oscillatory circuit being controllable tooscillate at two different natural frequencies.
 21. A transponderaccording to claim 20, wherein the control and transmission unit isarranged to control the application of voltage pulses with such afrequency that each of the two different natural frequencies correspondsto an integer multiple of the frequency at which the voltage pulses areapplied.
 22. A transponder according to claim 20, wherein the filter andoscillatory circuit comprises a first capacitor which is directly andpermanently electrically connected in parallel with the antenna and asecond capacitor which is arranged to be electrically connected anddisconnected, by a control signal from the control and transmissionunit, in parallel with the first capacitor.